Mangroves are coastal forests found in sheltered estuaries and along river banks and lagoons in the tropics and subtropics. The term “mangrove” describes both the ecosystem and the plant families that have developed specialized adaptations to live in this tidal environment [1]. Its multifaceted role, including the interactive relationship with the neighboring habitat and sheltering diverse species, has made it a treasured storehouse of the nature particularly production of fish and shellfish. Mangroves are one of the most productive ecosystems that enrich coastal waters, yield commercial forest products, protect coastlines, and even support coastal fisheries and storehouse of numerous endangered faunas (like Panthera tigris tigris, dolphin, otters, manatees and numerous avian species like egrets, pelicans, eagles) [2,3]. Estuaries are regions of enhanced biogeochemical activity and impart important ecosystem services along with supporting complex food webs [4]. Mangroves act as a fragile link between marine and fresh water ecosystems, pollution sink and source of nutrient flux into marine ecosystem. But, one is bound to be surprised to know that such a natural fighter against pollution is constantly being affected by the rising level of pollution. The aim of the review paper is to find out how this unique ecosystem, even if being adversely affected by pollution, still sustains the seminal balance of the ecosystem and plays a key role in nutrient cycling in coastal and estuarine ecosystem.

Mangrove restoration work has been carried out in the Tutuila Island, American Samoa [5]. They are in opinion that rehabilitation sites must meet the environmental conditions (e.g., duration, frequency and depth of inundation, wave energy, substrate conditions, salinity regime, soil and water pH, sediment composition and stability, nutrient concentrations, elevation, slope) required by mangrove species indigenous to the area.

Basically Mangroves are woody halophytic plants, which exist in the conditions of high salinity, extreme tides (Figure 1(a)), strong winds, high temperatures and muddy-anaerobic soils. The halophytic adaptations of mangroves, such as vivipery (Figure 1(b)), support roots (Figure 1(c)), negatively geotropic breathing roots (i.e., pneumatophores), sclerophyllous leaves with salt excretion glands and sunken stomata, stilt root and root but

tress are all indicative of the evolutionary selection to persist in muddy, brackish coastal environment on physiologically dry soil.

Mangroves are salt tolerant species and can take up water despite of high osmotic potential of soil water and even if the salt is absorbed, it is excreted through the salt glands in the leaves.

To mark out the general mangrove community along with other flora and fauna, the term “mangal” was proposed [6].

The exact number of species is still under discussion and ranges from 50 to 70 according to different classifications [1] with the highest species diversity found in Asia, followed by eastern Africa. In India, if all vascular plants are taken into consideration, east coast has 64 species (42 genera and 29 families) whereas the western coast has 33 species (24 genera and 19 families) and Andaman and Nicober Island has 43 species (30 genera and 23 families). About 60% of Indian mangroves are present in the east coast along Bay of Bengal, 20% in west coast along Arabian Sea and 13% on Andaman and Nicobar islands [7]. Mangrove forest cover in India is classified as very dense (>70% plant cover, 1405 sq km), moderately dense (40% - 70%, 1659 sq km), and open type (10% - 40%, 1575 sq km) forest types [7]. It is reported that Sunderbans alone has 62 species of mangroves [8]. The Indo-Malaysian region is considered to be the centre for the evolution of Mangrove vegetation [9].

Mangrove ecosystems can be used as indicators of coastal change or sea-level rise. These ecosystems are so specialized that any minor variation in their hydrological or tidal regimes causes noticeable mortality [10]. Mangrove ecosystem also serves as conservation of nutrients by storing them in dead roots ranging from 36% - 88% of total living tree biomass [11], unlike terrestrial forests where a large proportion of nutrient capital is stored in floor litter.

Unfortunately this unique ecosystem is itself being destructively harmed by the progress of civilization. Recent assessments on extent of mangroves worldwide suggests that between 1990 and 2010 there is a reduction of 3% of mangroves cover throughout the world and reasons are primarily land conversions for coastal development, rice production and aqua cultural projects [12]. However during 1980-2005, the aerial extent of mangrove forest loss is 30% - 50%, as a result of coastal development, aquaculture expansion, and over harvesting, which accounts for 36,000 km² [3]. Current extents of mangrove areas in different countries are represented in Figure 2.

About 25 countries of Asia have mangrove ecosystem, with climatic variation, ranging from arid (Arabian Peninsula) to sub-tropical (China, Japan) to humid tropical (South East Asia) [3]. Asia has the largest mangrove area in the world, with highest biodiversity of more than 50 species of true mangroves. Amongst them some are regional endemics like-Aegiceras floridum, Camptostemon philippinensis, Heritiera globosa. Kandelia candel a Rhizophoraceae member is found in north as far as Japan but is rare in SE Asia. High rainfall and substantial fresh

water input from rivers makes Bangladesh, India, Malaysia, Thailand and Indonesia a favorable place for growth of well structured mangroves, where the trees grow to a height of 30 - 50 m.

In Asia, Sundarbans, is the world’s largest contiguous mangrove patch covering an area of 10,000 km² and is the part of the progradation delta of Ganga-Brahmaputra-Meghna river systems that comprises of an area of 80,000 km² [13-15] and recognized internationally as the UNESCO (United Nations Educational, Scientific and Cultural Organization) World-Heritage site, The transboundary forest of Sundarbans is spread over two countries, of which 60% is in Bangladesh and 40% in India. This mangrove ecosystem is affected by numerous cyclonic storms [16]. But that is also to some extent threatened by the above mentioned problems. Asian mangroves as a whole is affected by anthropogenic disturbances like intensive logging, land conversion to promote paddy cultivation and aquaculture and pollution [3]. According to the estimate, since 1980, 25% loss of mangrove has been observed in Asia which was mainly due to intense deforestation activities [3]. So in current scenario the need of the hour is to conserve this fragile ecosystem. Though mangrove ecosystem is an important focus for conservation biologists, environmentalists but the growth of public consciousness to conserve mangrove ecosystem still remains as the burning question [17,18].

Like all other green species, Mangrove has got definite role against the pollution. It has natural ability to act as a sink of anthropogenic and industrial pollutants. Mangrove ecosystems are specific in numerous aspects (e.g. carbon and nutrients cycles, sediment characteristics, tidal conditions) which are expected to affect the speciation, and therefore the bioavailability of contaminants [19]. It can also arrest and bioremidiate certain pollutants (like fluoride) in local environment [20,21]. It not only acts as a sink or transfers the pollutants but also oxidizes the metals present in the sediment by exuding oxygen into the anoxic soil through aerial roots [22]. Mangrove wetlands are used for low cost waste disposal site [23,24]

Rise in industrialization and uncontrolled anthropogenic pressure on virgin mangrove patches has been increase in recent years, however, mangroves ecosystem adapted themselves by acting as natural pollution sink. Mangrove soils/sediments are usually fine-grained, water-logged and receive allochthonous organic matter from terrigenous origins [17]. Chemical contaminants in mangrove ecosystems are present between pore water, overlying water, and solid phases such as sediment, suspended particulate matter and biota [17]. According to the previous review work, the inundation of mangroves generally results in the depletion of oxygen in the organic rich sediments [19]. Since sulfate ions are usually present in large supply, sulfidic conditions will also arise. The stratification of redox conditions, from suboxic to anoxic and sulfidic, was reported for unvegetated sediments and those covered with mangrove plants. In the sulfidic zones, the co-precipitation of trace metals together with other sulfide minerals (e.g. iron sulfide) is described as a major process leading to the immobilization of metals in mangroves. Physico-chemical changes in the rhizosphere are also seen to be associated with changes in the concentration and the speciation of trace metals [19].

Carbon emission is the major cause for climate change and global warming. There is a 36% raise in global CO₂ emission from 1992 to 2008 and it shows a steady mean increase from 357 ppmv (parts per million by volume) in 1992 to 389 ppmv in 2011 [12]. Mangroves act as an effective carbon sink [37] and sequester higher amount of CO₂ (than any other non-mangrove forest types) which approximately amounts to 100 tons of CO₂ per hector (Ha) and also stabilize the soil particles to control erosion [38]. Mangroves are most carbon-rich forests in the tropics and well known for high carbon assimilation and flux rates. Another estimate elucidates that coastal and estuarine mangrove forests retains 1023 Mg carbon per hectare [39]. While, comparing the carbon storage capacity (both above and below ground) of mangroves ecosystem with other ecosystems, mangroves has superiority and could be act as effective carbon sink. Decomposing of mangrove litter is rapid in wet soil which adds to the carbon flux of the soil thereby increasing the total organic carbon of the soil. Organic rich soils ranged from 0.5 m to more than 3 m in depth and accounted for 49% - 98% of carbon storage [39]. Mangrove deforestation generates emission of 0.02 - 0.12 Pg carbon/year, as much as around 10% of emissions globally [39] despite accounting for about 0.7% of tropical forest area [40].

It will be a mere repetition if we again elaborate or point out the multilayered function of this unparallel ecosystem. Therefore protection and conservation of mangrove ecosystem in the developing countries should be given priority. Along with the above factors, chemical pollution, particularly accumulation and bio-transformation of toxic metals could be a significant factor for reduction of mangrove biodiversity. Mangrove sediment is soft and muddy, extended up to a depth of 3 - 4 m, rich in organic carbon, and acted as a “carbon reservoir”. The assessment of distribution of carbon profile (both spatial and temporal) is essential to determine the C-sequestration potential of mangrove ecosystem for developing countries.

The authors are grateful to Ms. Namrata Dey Roy, (Lecturer Susil Kar College, University of Calcutta) for helping in editing the manuscript.